Compact and efficient power supplies are an increasing concern to users and manufacturers of electronics. Switching power supplies with pulse width modulated ("PWM") controllers offer both compactness and efficiency in a number of different topologies. Boost and buck switching power supply topologies are efficient, but do not isolate the power input from the power output. Other topologies, such as the flyback, do isolate the power input from the power output by using a transformer. In such topologies, feedback from the secondary (power output) side of the transformer is needed to adjust the pulse width modulation duty cycle of the power switch. PWM control for a switching power supply may be provided from a single integrated circuit chip or package having some number of external connection pins or terminals. As with many other types of integrated circuit chips or packages, limiting the number of external connection terminals of a power supply package can be advantageous.
For example, U.S. Pat. No. 5,313,381 to Balakrishnan (the "'381 patent"), which is fully incorporated by reference, discloses a three-terminal switching power supply control chip for use with a flyback converter. FIG. 1 illustrates a flyback converter 20 according to the '381 patent. The converter 20 employs a three-pin control chip 22 to supply current from a rectified DC source (Vbb) 28 across an isolating transformer 24 to supply power for a load 26. The power supply chip 22 includes a first terminal 30 coupled to a primary winding 32 of the transformer 24, a second ("ground") terminal 36 coupled to a primary side ground reference, and a third terminal 40 for accepting a combined feedback control signal (IFB) and a bias supply voltage (Vcc) to operate the control chip 22.
Within the power supply chip 22, the first terminal 30 is alternately coupled to the ground terminal 36 by a power transistor switch 42. PWM control circuitry 44 drives the power switch 42 at a variable duty cycle. When the power switch 42 is ON, current flows through the primary winding 32 and energy is stored in the magnetic core 45 of the transformer 24. When the switch 42 is OFF, a secondary diode 46 is forward biased and the stored energy in the transformer core 45 is released through a secondary winding 48 to a filter/storage capacitor 47 and the load 26. After the transformer 24 is reset, the ON/OFF cycle is repeated.
An error amplifier 50 compares the output voltage Vout across the load 26 with a reference voltage to generate the feedback control signal IFB. The bias supply voltage Vcc is supplied from an auxiliary secondary winding 52 of the transformer 24. The bias supply voltage Vcc is modulated with the feedback control signal IFB in an opto-isolator 54 to create the combined bias voltage, feedback signal Vcc/IFB. A feedback extraction circuit (not shown) in the chip 22 separates the feedback signal IFB from the bias voltage Vcc by sensing the excess current flowing through a shunt regulator. The extracted feedback signal IFB is used to control the output of the PWM circuitry 44 to constantly adjust the duty cycle of the power switch 42 so as to transfer greater or lesser current to the secondary.
While the flyback converter taught by the '381 patent provides certain advantages, it would be desirable to provide minimal terminal power supply packages for other types of converter topologies, including multi-switch controlled power converter topologies.